1. Field of the Invention
The present invention relates to a semiconductor laser device having a layered structure which is lattice-matched with a substrate. The semiconductor laser devices referred to in this specification may include semiconductor laser diodes, semiconductor optical amplifying devices, and any other semiconductor laser devices.
2. Description of the Related Art
Currently, short-wavelength semiconductor laser devices using GaN materials and II-VI group materials are being studied. For example, Japanese Journal of Applied Physics, Vol. 37 (1998) pp.L309-L312 reports a semiconductor laser device which is constructed by forming, on a GaN substrate, an n-type GaN buffer layer, an n-type InGaN crack preventing layer, an n-AlGaN/GaN modulation-doped superlattice cladding layer, an n-type GaN optical waveguide layer, an n-InGaN/InGaN multiple quantum well active layer, a p-type AlGaN carrier block layer, a p-type GaN optical waveguide layer, an p-AlGaN/GaN modulation-doped superlattice cladding layer, and a p-type GaN contact layer, where the GaN substrate is formed by selective growth using a mask of an SiO2 film on a GaN layer formed on a sapphire substrate. Oscillation of the 410 nm band is realized by the above semiconductor laser device.
However, there is a great degree of lattice mismatching (i.e., a great strain) between the GaN substrate and the active layer in the above construction of layers. Therefore, it is impossible to increase the indium content in the active layer, and thus conventional semiconductor laser devices which operate at wavelengths equal to or longer than 450 nm are not reliable.
The strain between the grown layer and the substrate is defined as (axe2x88x92as)/as, where as denotes a lattice constant of the substrate, and a denotes a lattice constant of the grown layer. Generally, the xe2x80x9clattice matchingxe2x80x9d is defined by the condition that the strain is not less than xe2x88x920.01 and not greater than 0.01.
As described above, in order to obtain reliable semiconductor laser devices which oscillate at wavelengths equal to or longer than 450 nm, it is necessary to increase the indium content in the active layer. However, conventionally, it is difficult to realize such semiconductor laser devices due to the increase in the strain because the great strain between the grown layers and the substrate is liable to cause cracking or dislocation. Therefore, reliability of the conventional semiconductor laser devices is decreased in high power operations.
In addition, in semiconductor laser devices, it is desirable to thicken cladding layers to reduce overflow currents and optical losses. However, conventionally, it is also impossible to realize thick cladding layers due to the above great strain, which is liable to cause cracking.
The object of the present invention is to provide a short-wavelength semiconductor laser device which is reliable in high power operations.
The object of the present invention is accomplished by the present invention, which provides a semiconductor laser device which contains a conductive substrate connected to one of a pair of electrodes, a lower cladding layer, a lower optical waveguide layer, a single or multiple quantum well active layer, an upper optical waveguide layer, an upper cladding layer, a contact layer, and the other of the pair of electrodes, which are stacked in this order, wherein the conductive substrate is made of InGa material, and the lower cladding layer has a composition which causes a strain which is not less than xe2x88x920.01 and not greater than 0.01 between the lower cladding layer and the conductive substrate, and is made of one of InGaN and InGaAlN material.
The above conductive substrate may be formed on a first substrate which is formed on a second substrate by selective growth, where the first substrate may be made of InGa material.
Since the InGaN substrate is used, instead of the conventional GaN substrate, the range of compositions realizing lattice matching is extended. Therefore, it becomes possible to use InGaAlN material for the cladding layer so that the cladding layer lattice-matches the InGaN substrate. Hence, generation of cracking or dislocation can be prevented, and the indium content in the active layer can be increased. Thus, high-power long-wavelength oscillation up to 550 nm can be realized.
In addition, since generation of cracking is prevented by the lattice-matching between the cladding layers and the substrate, it is possible to realize cladding layers having thickness equal to or more than one micrometer, which is sufficient to reduce the amounts of overflow currents and optical losses in the optical waveguide. Thus, reliability is increased.
In the above semiconductor laser device according to the present invention, the contact layer may be made of InGaN material. In this case, the contact resistance can be reduced, and therefore temperature increase in high power operations can be reduced. Therefore, reliability of the semiconductor laser device in high power operations is increased.
Since, according to the present invention, all or almost all of the layers above the InGaN substrate of the semiconductor laser device include indium, the number of changes in growth temperature during the formation of the layers can be reduced. Therefore, it is possible to reduce growth interruption time, which is necessary for raising or lowering the growth temperature, and it is also possible to reduce probability of defect generation during the growth interruptions.
In the above semiconductor laser device according to the present invention, the cladding layers may have a superlattice structure, and compositions of the cladding layers may be such that the strain in the cladding layer is not less than xe2x88x920.01 and not greater than 0.01.
In addition, the above cladding layers may have a modulation-doped superlattice structure in which impurity is doped into barrier layers in the superlattice structure, and compositions of the cladding layers may be such that the strain in the cladding layer is not less than xe2x88x920.01 and not greater than 0.01 . Further, the cladding layers may have a superlattice structure in which impurity is doped into both of the well layers and the barrier layers in the superlattice structure.
Further, preferably, the first substrate may be one of sapphire, SiC, ZnO, LiGaO2, LiAlO2, ZnSe, GaAs, GaP, Ge, and Si.